# std::stable_sort

From cppreference.com
< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class RandomIt > void stable_sort( RandomIt first, RandomIt last ); | (1) | (constexpr since C++26)
template< class ExecutionPolicy, class RandomIt > void stable_sort( ExecutionPolicy&& policy, RandomIt first, RandomIt last ); | (2) | (since C++17)
template< class RandomIt, class Compare > void stable_sort( RandomIt first, RandomIt last, Compare comp ); | (3) | (constexpr since C++26)
template< class ExecutionPolicy, class RandomIt, class Compare > void stable_sort( ExecutionPolicy&& policy, RandomIt first, RandomIt last, Compare comp ); | (4) | (since C++17)

Sorts the elements in the range `[`first`,` last`)` in non-descending order. The order of equivalent elements is guaranteed to be preserved.

1) Elements are sorted with respect to operator<(until C++20)std::less{}(since C++20).
3) Elements are sorted with respect to comp.
2,4) Same as (1,3), but executed according to policy.
These overloads participate in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. | (since C++20)

If any of the following conditions is satisfied, the behavior is undefined:

The type of *first is not Swappable. | (until C++11)
`RandomIt` is not ValueSwappable. The type of *first is not MoveConstructible. The type of *first is not MoveAssignable. | (since C++11)

### Parameters

first, last | - | the range of elements to sort
policy | - | the execution policy to use
comp | - | comparison function object (i.e. an object that satisfies the requirements of Compare) which returns ​true if the first argument is less than (i.e. is ordered before) the second. The signature of the comparison function should be equivalent to the following: bool cmp(const Type1& a, const Type2& b); While the signature does not need to have const&, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) `Type1` and `Type2` regardless of value category (thus, `Type1&` is not allowed, nor is `Type1` unless for `Type1` a move is equivalent to a copy(since C++11)). The types Type1 and Type2 must be such that an object of type RandomIt can be dereferenced and then implicitly converted to both of them. ​
Type requirements
- `RandomIt` must meet the requirements of LegacyRandomAccessIterator.
- `Compare` must meet the requirements of Compare.

### Complexity

Given \(\scriptsize N\)N as last - first:

1,2) \(\scriptsize O(N \cdot \log(N))\)O(N·log(N)) comparisons using operator<(until C++20)std::less{}(since C++20) if enough extra memory is available, otherwise \(\scriptsize O(N \cdot \log^{2}(N))\)O(N·log2
(N)) comparisons.
3,4) \(\scriptsize O(N \cdot \log(N))\)O(N·log(N)) applications of the comparator comp if enough extra memory is available, otherwise \(\scriptsize O(N \cdot \log^{2}(N))\)O(N·log2
(N)) applications.

### Exceptions

The overloads with a template parameter named `ExecutionPolicy` report errors as follows:

- If execution of a function invoked as part of the algorithm throws an exception and `ExecutionPolicy` is one of the standard policies, std::terminate is called. For any other `ExecutionPolicy`, the behavior is implementation-defined.
- If the algorithm fails to allocate memory, std::bad_alloc is thrown.

### Possible implementation

See also the implementations in libstdc++ and libc++.

### Notes

This function attempts to allocate a temporary buffer equal in size to the sequence to be sorted. If the allocation fails, the less efficient algorithm is chosen.

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_constexpr_algorithms` | `202306L` | (C++26) | constexpr stable sorting, overloads (1), (3)

### Example

Run this code

    #include <algorithm>
    #include <array>
    #include <iostream>
    #include <string>
    #include <vector>

    struct Employee
    {
        int age;
        std::string name; // Does not participate in comparisons
    };

    bool operator<(const Employee& lhs, const Employee& rhs)
    {
        return lhs.age < rhs.age;
    }

    #if __cpp_lib_constexpr_algorithms >= 202306L
    consteval auto get_sorted()
    {
        auto v = std::array{3, 1, 4, 1, 5, 9};
        std::stable_sort(v.begin(), v.end());
        return v;
    }
    static_assert(std::ranges::is_sorted(get_sorted()));
    #endif

    int main()
    {
        std::vector<Employee> v{{108, "Zaphod"}, {32, "Arthur"}, {108, "Ford"}};

        std::stable_sort(v.begin(), v.end());

        for (const Employee& e : v)
            std::cout << e.age << ", " << e.name << '\n';
    }

Output:

    32, Arthur
    108, Zaphod
    108, Ford

### See also

sort | sorts a range into ascending order (function template)
partial_sort | sorts the first N elements of a range (function template)
stable_partition | divides elements into two groups while preserving their relative order (function template)
ranges::stable_sort (C++20) | sorts a range of elements while preserving order between equal elements (algorithm function object)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/algorithm/stable_sort&oldid=171970"